A type of sensor for detecting the temperature of a gaseous fluid is, for example, a hot-film air mass meter (HFM), which is described, for example, in German Patent Application No. DE 196 01 791. In hot-film air mass meters of this type, a thin sensor diaphragm is typically applied to a sensor chip, which is preferably a silicon sensor chip. At least one heating resistor, which is enclosed by two or more temperature measuring resistors, is typically situated on the sensor diaphragm. The temperature distribution which may in turn be detected by the temperature measuring resistors changes in an air flow which is guided over the diaphragm. Thus, for example, an air mass flow may be determined from the resistance differential of the temperature measuring resistors. Various other variations of this sensor type are known from the related art. A sensor chip, which has a frame element manufactured from silicon, having a diaphragm applied thereto, is described in German Patent No. DE 101 11 840. Various metal webs which function as electrical heaters and/or measuring resistors are situated on the diaphragm, resulting in the area of the diaphragm forming a sensor area. Moreover, at least one auxiliary heater which may be electrically heated in such a way that thermal gradient eddies are formed in the flowing medium in the area of the auxiliary heater may additionally be situated on the surface of the sensor chip.
In addition to the detection of the temperature of a gaseous fluid, the detection of components of which the particular gaseous fluid is composed plays a large role. To detect hydrogen, for example, the property of hydrogen of having significantly better thermal conductivity than air, for example, is exploited. In a sensor construction which has a similar design to that of hot-film air mass meters (HFM), an air-hydrogen mixture, for example, diffuses through a thin diaphragm or a tight grid into the measuring chamber of a sensor. The presence of hydrogen in the gaseous fluid changes the temperature of the heated measuring diaphragm or its thermal output, which is delivered to the surrounding air. A measurement signal is in turn generated therefrom. These sensors are typically operated at measuring chip and/or housing temperatures which approximately correspond to room temperature (25° C.). The diaphragms used in the sensors are typically operated at higher temperatures than the measuring chip or housing temperature, between 80 K and 120 K. These measuring sensors have the disadvantage of the moisture component contained in the gaseous fluid. The moisture contained in the gaseous fluid influences the thermal conductivity of the gaseous fluid, e.g., a hydrogen-air mixture. At room temperature, approximately 25° C., the influence of the moisture component contained in the gaseous fluid may be so large that detection of hydrogen by the sensor is no longer possible with the required clarity.
In diffusive processes, the thermal conductivity of the gaseous fluid dominates the heat transmission, while in conductive heat exchange processes, i.e., when fresh air is supplied, the heat capacity of the gaseous fluid is decisive for the resulting heat transmission. The atmospheric moisture contained in the gaseous fluid influences both heat transmission mechanisms, the thermal capacity of the gaseous fluid rising and its thermal conductivity sinking in the event of increasing moisture. The heat transmission and thus the heat flow are influenced in accordance with the proportions of diffusion and convection in the heat transmission process.